1. Field of the Invention
The present invention relates generally to the field of measuring apparatus for light masses. More particularly, the present invention relates to springs used in a compact electronic balance capable of correcting an error in the measured value due to off-center placement of the mass to be weighed.
2. Discussion of the Prior Art
There are many ways to measure weight. One problem that all weighing apparatus face is that of off-center pan loading in which the mass to be weighed is not located at the exact center of the weighing pan, but is off to one side. In general, when a load is placed deviatedly from a required point of placement, such as the center of the pan, the bending moment exerted on the pan thereby produces an error in the measured value. Until recently, there were two basic ways of handling the problem of off-center pan loading: the "hanging pan" and the "parallel-motion" techniques. The "hanging pan" technique consists of a pan suspended from a knife edge support at a single point, such that gavity will twist the pan so as to locate its center of gravity directly below the pan support. This prevents bending moments in the load sensing mechanism. With the "parallel-motion" technique, a parallel-motion linkage is attached to the pan. This absorbs the bending moment caused by off-center loading and prevents bending moment forces from being applied to the load cell.
The hanging pan technique is mechanically simple with a simple sensor. It has a very tall vertical scale dimension, but no adjustments for off-center loading are required. However, friction at the knife-edge and overall inertia are sources of error when weighing very light masses. Devices constructed in accordance with this technique tend to be bulky, and the pan's hanger is often in the way when loading. Levelling of the overall device is important.
Devices constructed in accordance with the parallel-motion technique are mechanically complex. They are top-loading and have a thick, but simple sensor. The vertical scale dimension is less than that of a hanging pan but is still substantial. Adjustment for off-center loading is required through mechanical adjustments of the linkage. Misadjustment of the linkage is a common source of error. Levelling of the device is very important.
U.S. Pat. No. 4,738,324 to Borchard, the specification of which is hereby incorporated by reference, discloses a third, improved way of handling the problem of off-center pan loading. The technique of that system is a self-adjusting one in which the effects of off-center loading in the load cell are measured and corrected before the weight reading is outputted. The device constructed in accordance with that technique measures deflections at a plurality of locations on a spring-supported pan, and combines the separate measurements to yield the weight on the pan, corrected for load placement location, i.e., off-center loading. The deflection measurements are accomplished using a plurality of capacitors formed between two rigid, closely-spaced plates with pie-shaped patterns on the plates forming the capacitors. These are not actually three separate capacitors, but one device acting like three different capacitors and/or performing as if it were three separate capacitors. The plates are glued to the weight bearing pan and a base. The plates are held apart by one or more leaf springs spaced around the periphery of the plates. When an object to be weighed is placed on the weighing pan, circuitry converts the capacitance measured for each capacitor into a frequency that is then used in conjunction with preset adjustment factors, to determine the weight of the object and to compensate for any off-center loading of the weighing pan.
Although the system of the '324 patent provided numerous advantages over the prior art, it required careful assembly to form the three leaf springs which created the displacement between the plates. Additionally, having the three leaf springs protruding from the periphery of the plates resulted in an assembled pan and sensor assembly that was relatively large. Other examples of prior art leaf springs for use in scales for weighing light masses may be seen in U.S. Pat. Nos. 4,828,057; 4,869,331; and 4,874,051.
In the arrangement of the '324 pan and sensor assembly, the gap between the plates is a function of both the thickness of the disks and the opening in the springs. Therefore, thermal expansion of the disks and changes in the spring temperature can cause significant changes in the dimension of the gap separating the plates thus adversely affecting measurement accuracy and particularly requiring extended periods of time to allow the pan and sensor assembly to stabilize after being subjected to a change in temperature.
Other aspects of prior art weighing systems may be seen in U.S. Pat. Nos. 4,862,978 and 4,862,979.
Therefore, an object of the present invention is to provide an improved version of the '324 weighing system which has a more compact construction.
Another object of the present invention is to provide a weighing system using a pan and sensor assembly that can be manufacture using simpler assembly techniques than in the prior art.
Yet another object of the present invention is to provide springs for use in a balance having improved temperature characteristics and having a more compact construction than the springs of the prior art.